Influences of thermal radiation on the heat transfer of a laminar flow semi-transparent fluid in a cylindrical tube

By numerically simulating the coupled radiation and convection heat transfer of a semi-transparent fluid in a circular tube, the influences of thermal radiation on temperature distribution and convective heat transfer are investigated. The fluid, which is an absorbing, emitting and isotropic scattering gray medium with constant properties, flows in a fully developed laminar flow state. The nonlinear integro-differential radiative transfer equation in the cylindrical coordinate system is solved by the discrete ordinate method to obtain the radiative heat transfer term, which is dealt with as a source term in the energy equation of the coupled heat transfer. With the given velocity distribution of developed laminar flow in tube, the energy equation is solved by the control volume method. Under the constant wall temperature condition, the influences of the medium radiation-conduction parameter and scattering albedo on the dimensionless temperature distribution and the local Nusselt number of convection are discussed under the different Peclet number and optical thickness. The results show that thermal radiation has significant effects on the heat transfer characteristics of a laminar flow semitransparent fluid in a circular tube. In addition to accelerate the growth of the temperature field, thermal radiation also make important changes to the local Nusselt number of convection. Compared with the corresponding convection heat transfer not accompanied by thermal radiation, in a coupled radiation and convection heat transfer, the local Nusselt number of convection first descends within a region near the onset position of heating. Then it gradually increases along the flowing path until it reaches a higher value and keeps constant with the thermal boundary layer being fully developed. By comparing the two conditions of heating (medium being heated) and cooling tubes, it is found that the effect of thermal radiation in coupled heat transfer is more obvious in a cooling tube than that in a heating tube.